JPH07294971A - Production of second harmonic wave generating element - Google Patents

Abstract

PURPOSE:To form deep polarization inversion regions in a single crystal by forming the specific polarization inversion regions by an ion exchange method. CONSTITUTION:The polarization inversion regions 3 are formed to a comb shape at a depth of 50 to 500mum depth from the surface on the minus(z) surface of a single crystal z plate 5 consisting of KTP by executing ion exchange by an ion exchange method in a fused salt mixture contg. barium and potassium or barium and rubidium under conditions of time from 30 minutes to 4 hours and 250 to 500 deg.C. The compsn. of the parts subjected to the polarization inversion produced in such a manner is so formed as to consist of K1-x BaxTiOPO4 or K1-x-y BaxRbyTiOP04. The deep polarization inversion layers are formable in the single crystal of the KPT by using this method. The polarization inversion layers are first formed in the single crystal of the KTP and thereafter, optical waveguides 2 are formable at the desired depth. As a result, the second harmonic wave generating element provided with the high conversion efficiency by the presence of segment type optical waveguides lowered in the scattering loss of light and the sufficiently deep polarization inversion layers is produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of optical information processing using laser light, the field of optical applied measurement control, the field of printing / platemaking,
The present invention relates to a method for manufacturing an optical wavelength conversion element used in the medical field or the like, particularly a second harmonic generation element.

[0002]

2. Description of the Related Art In order to efficiently convert the wavelength of light into a second harmonic, it is necessary to satisfy the phase matching condition in the wavelength conversion element. As this method, angle phase matching, temperature phase matching, a method using a waveguide, etc. have been proposed and used conventionally. Recently, a method called quasi-phase matching using a periodic structure has attracted attention as a phase matching method. For example, Phys. Rev., Vol. 127, p. 1918 (196
2) by JA Armstrong et al.
In this method, the polarization direction in the crystal is periodically inverted to compensate for the phase mismatch between the fundamental wave and the harmonic.

Here, the polarization reversal means reversing the direction of polarization of the single crystal dielectric material polarized in a fixed direction to be used. A polarization inversion type SHG element is obtained by applying this method to a second harmonic generation (SHG) element.

Various methods have been used so far for causing this polarization inversion. For example, Lim EJ, Fejer
MM, Beyer RL and Kozolovsky WJ: Electron. Le
tt., 25, 11, pp. 731 (1989), thermal diffusion of Ti metal is used in lithium niobate (LN) single crystals. For example, K. Mizuuchi et al .: Appl. Phys. Let
As in t., 58, p. 2732 (1991), a method of rapid heating after proton exchange is used in lithium tantalate (LT) single crystal. As shown in FIG. 1, LN or L
In the T-crystal substrate 1, the domain-inverted regions 3 and the non-domain-inverted regions 4 are formed in a comb shape, and the channel waveguide 2 is formed in the direction orthogonal to the pattern to form a domain-inverted SHG element. FIG. 1 shows the structure of this SHG element.

Further, as shown in FIG. 2, in the KTP substrate 5, the segment type optical waveguide 2 is formed by the ion exchange method by immersing in the molten salt of rubidium nitrate.
A barium nitrate is added to the molten salt at the same time as the production of the above, and the polarization inversion of that portion is performed to produce a polarization inversion type SHG element (Appl. Phys. Lett.,
Vol.57, No.20, p.2074 (1990)). FIG. 2 shows the structure of this SHG element. Here, in FIG. 2, 3 is a polarization inversion region, and 4 is a non-polarization inversion region.

So far, we have been able to solve the problem of leaked light, which has been a problem in segment type waveguides in KTP, by forming each segment into a lens shape (JP-A-5-164017).

Recently, polarization inversion is also performed in LT, LN and KTP by applying a DC voltage (for example, Sawaki, Miura, Kurimura: 199).
Fall 2nd year, Japan Society of Applied Physics, 18a-X-2).

[0008]

When the KTP crystal is used and the ion exchange method is used, if the polarization can be inverted up to a deep region (50 to 500 μm) inside the KTP crystal, it can be used as a quasi phase matching type element as a bulk.

Further, in the case of using the KTP crystal, in the segment type, if each segment is a square type, light leaks to both the incident fundamental wave and the generated second harmonic in principle due to scattering in the Fresnel region. coming out. As a result, the conversion efficiency was lowered. Moreover, since the second harmonic is also scattered, the phenomenon that the outgoing beam cannot be narrowed down to the diffraction limit is also caused.

An object of the present invention is to provide a second harmonic generation element using KTP, which can form a deep (50 to 500 μm) domain-inverted region in a single crystal by an ion exchange method. A method for manufacturing the element, and a second harmonic generation element with a segment type optical waveguide perpendicular to the domain-inverted and non-domain-inverted regions produced by the ion exchange method and having no leakage light can be produced by the ion exchange method. It is an object of the present invention to provide a method of manufacturing a second harmonic generation element that can be performed.

[0011]

In order to solve the above problems, it was discovered that a deep polarization inversion region can be formed in KTP by an ion exchange method. Further, by forming a segment type optical waveguide at right angles to the domain-inverted region, that is, by making the shape of the segment a lens, it is possible to reduce the light scattering loss that occurs when each segment is a square type. I discovered that it can be made. Specifically, as shown in FIG. 3, on the minus z plane of the KTP single crystal z plate 5, a non-polarization inversion region and a polarization inversion region having a higher refractive index than the lens type bulk in a segment shape, or the lens type bulk. By alternately forming polarization inversion regions and non-polarization inversion regions with a higher refractive index, the laser light emitted from one ion-exchanged segment is focused by the refractive index distribution of that segment, resulting in light scattering. It was discovered that a structure that can reduce loss can be manufactured. In addition, since the refractive index distribution due to this ion exchange is graded in the depth direction, it was discovered that light confinement in the depth direction also becomes a lens type and a structure with very little leakage light is taken. At this time, the shape of the segment can be various shapes such as an ellipse and a circle as shown in FIG. Conventionally, polarization inversion and formation of an optical waveguide were performed simultaneously by ion exchange, but since polarization inversion is performed deeply first and an optical waveguide with a high refractive index is formed later, the optical waveguide depth That is, it surely contributes to the wavelength conversion of light. However, when the shape of the segment is a rectangle, the structure is the same as the conventional one, but the waveguide manufacturing method is different. The segment interval may be set to a value for the fundamental wave calculated by quasi phase matching. With respect to the width of the segment, it is possible to select a portion where a required value of light conversion efficiency appears, within 1 to 12 μm.

The method of polarization reversal is achieved by forming a comb-shaped metal mask on the minus z surface of KTP and immersing it in barium and potassium or a mixed molten salt of barium and rubidium for a long time, for example, for 30 minutes to 4 hours. To be done. The composition of the ion-exchanged portion produced at this time was K _1-x Ba _x Ti.
OPO _{4 or, K 1-xy Ba x Rb} y TiOPO 4
Becomes As the mask metal, it is possible to appropriately use a mask pattern forming metal such as Ti, Al, Ta, Ni, Cr, or an alloy thereof, depending on the ion exchange source used.

After the formation of the domain-inverted structure, a segment-type lens-shaped optical waveguide is formed perpendicular to the domain-inverted structure.
The period and shape of the lens may be produced according to the condition of the refractive index change during ion exchange.

It is also possible to simultaneously perform polarization reversal and optical waveguide fabrication as follows. A mask pattern is formed in the shape of a segment type lens, and polarization reversal is performed by immersing the mask pattern in barium and potassium or a molten salt of barium and rubidium for a long time, for example, for 30 minutes to 4 hours. The distance between the non-polarization-inverted region and the polarization-inverted region may be set to a value for the fundamental wave calculated by quasi-phase matching. After that, when polarization is inverted in the same region with a refractive index higher than that of the ion exchange treatment during polarization inversion, that is, when polarization inversion is performed with a mixed molten salt of barium and potassium, barium and rubidium or barium and thallium or barium and thallium and rubidium or When ion exchange is performed with a mixed molten salt of barium, thallium, and potassium, or when polarization is inverted with a mixed molten salt of barium and rubidium, ions are generated with a mixed molten salt of barium and thallium, barium and thallium, rubidium, and barium, thallium, and potassium. Replace and form the optical waveguide.

[0015]

When the above method is used, a deep polarization inversion layer can be formed on the KTP single crystal. Further, it is possible to first form a deep polarization inversion layer on the KTP single crystal and then form an optical waveguide to a desired depth. As a result, it becomes possible to fabricate a second harmonic generation device that can be used in bulk by the ion exchange method, and in the segment type optical waveguide with reduced light scattering loss, the conversion efficiency can be improved by the presence of a sufficiently deep polarization inversion layer. A high second harmonic generating element can be manufactured.

[0016]

EXAMPLES Examples according to the present invention will be described below. (First Example) KTP was used as a substrate. A resist was spin-coated on the minus z surface of this substrate, and comb-shaped patterning of the resist was performed by photolithography. This period is 4 microns. After Ti sputtering, the resist was removed to form a Ti comb pattern. Immerse in a molten salt (450-500 ° C.) of a mixture of potassium nitrate and barium nitrate in a ratio of 80:20 (other salt may be used) for 2 hours to obtain a non-polarization inversion region (2 μm length) and a polarization inversion region (2 μm length). Were alternately manufactured at a depth of 150 μm. After polishing the end face, the laser light was focused from the end face and introduced. A second harmonic of 1 nW was obtained for 10 mW of 861 nm laser light.

(Second Embodiment) A polarization inversion type S according to the present invention.
A perspective view of the HG element is shown in FIG. The substrate 5 uses KTP. A resist was spin-coated on the minus z surface of the substrate 5, and the resist was comb-shaped patterned by photolithography. This period is 4 microns. After Ti sputtering, the resist was removed to form a Ti comb pattern. 8 of potassium nitrate and barium nitrate
Molten salt of a mixture of 0 to 20 (other salts may be used) (45
Soaking in 0-500 ° C for 2 hours, non-polarization inversion region (2μm
Length) 4 and polarization inversion region (2 μm length) 3 150 μm
Were alternately manufactured at a depth of. After that, the Ti film is removed, and a mask pattern of Ti is formed into a lens shape by a photolithography process again. By dipping in a molten salt of rubidium nitrate and barium nitrate (8: 2) (350 ° C.) for 3 minutes, the optical waveguide 2 is formed on the surface portion, and a polarization inversion type SHG element is completed. The portion where the refractive index has changed due to ion exchange is a portion 10 to 25 μm from the crystal surface. Laser light was introduced after polishing the end faces. 863nm laser light 10mW
, The second harmonic of 1 μW was obtained.

Third Embodiment A polarization inversion type S according to the present invention
A perspective view of the HG element is shown in FIG. The substrate 5 uses KTP. A resist was spin-coated on the minus z surface of the substrate 5, and the resist was patterned into a segment lens shape by photolithography. This period is 4 microns. After Ti sputtering, the resist was removed to form a Ti lens pattern. Immerse in a molten salt (450-500 ° C.) of a mixture of 80:20 potassium nitrate and barium nitrate (other salts may be used) for 2 hours,
Non-polarization inversion area (2 μm length) and polarization inversion area (2 μm)
(Length) 3 were produced alternately. After that, it is immersed in a molten salt of rubidium nitrate and barium nitrate (8: 2) (350 ° C.) for 3 minutes to form an optical waveguide 2 on the surface portion, and a polarization inversion SH
G element can be formed. The portion where the refractive index has changed due to ion exchange is a portion 10 to 25 μm from the crystal surface. Laser light was introduced after polishing the end faces. A second harmonic of 1 μW was obtained for 10 mW of 860 nm laser light.

(Fourth Embodiment) A polarization inversion type S according to the present invention.
A perspective view of the HG element is shown in FIG. The substrate 5 uses KTP. A resist was spin-coated on the minus z surface of the substrate 5, and the resist was patterned into a segment lens shape by photolithography. This period is 4 microns. After Ti sputtering, the resist was removed to form a Ti lens pattern. Immersed in a molten salt (350 ° C.) of a mixture of rubidium nitrate and barium nitrate of 80 to 20 (other salt may be used) for 2 hours, and a non-polarization inversion region (2 μm length) and a polarization inversion region (2 μm length)
3 was alternately produced at a depth of 150 μm. Then, it is immersed in a mixed molten salt (8: 2: 1) of rubidium nitrate, barium nitrate, and thallium nitrate (350 ° C.) for 3 minutes to form the optical waveguide 2 on the surface portion, thereby forming an optical wavelength conversion element. The area where the refractive index has changed due to ion exchange is 10
˜25 μm portion. Laser light was introduced after polishing the end faces. 1μ for 10mW of 860nm laser light
The second harmonic of W was obtained.

[0020]

As described above, according to the present invention, K
In the TP single crystal, a polarization inversion layer deeper than the surface can be formed by an ion exchange method, and further, a second harmonic generating element in which the optical waveguide has a desired mode number (depth) can be manufactured by the ion exchange method.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a conventional domain-inverted SHG device in lithium niobate (LN).

FIG. 2 is a schematic diagram of a conventional polarization inversion SHG element in KTP.

FIG. 3 is a schematic view of a domain-inverted SHG element according to the present invention.

FIG. 4 is a mask diagram of various segment patterns.

FIG. 5 is a schematic view of a domain-inverted SHG element according to the present invention.

FIG. 6 is a schematic view of a domain-inverted SHG element according to the present invention.

[Explanation of symbols]

 1 LN or LT substrate 2 Optical waveguide 3 Polarization inversion region 4 Non-polarization inversion region 5 KTP substrate

Claims (2)

[Claims] 1. A negative molten z-plane of a KTP single crystal z plate is subjected to ion exchange in a mixed molten salt containing barium and potassium or barium and rubidium by an ion exchange method for 30 minutes to 4 hours at a temperature of 250 to 500 ° C. Then, a domain-inverted region was formed in a comb shape at a depth of 50 to 500 μm from the surface, and the composition of the produced domain-inverted portion was K.
_1-x Ba _x TiOPO ₄ or K _1-xy Ba _x Rb
_A method of manufacturing a second harmonic generation element, characterized in that it is _y TiOPO ₄ . 2. A mixed molten salt containing barium and potassium or barium and rubidium when a domain-inverted region is formed in a lens shape at a depth of 50 to 500 μm from the surface on the minus z surface of a KTP single crystal z plate. Ion exchange by ion exchange method for 30 minutes to 4 hours at a temperature of 250
The composition of the polarization-inverted portion produced under the condition of up to 500 ° C. is K _1-x Ba _x TiOPO ₄ or K _1-xy Ba
and _x Rb _y TiOPO _4, further becomes higher conditions of a refractive index higher than the ion exchange process when poled the same region, when poled in a mixed molten salt of the so-called barium and potassium, Ya barium rubidium, barium and thallium When ion-exchanged with a mixed molten salt of barium, thallium, rubidium or barium, thallium and potassium, or when polarization inversion was performed with a mixed molten salt of barium, rubidium, barium and thallium, barium and thallium and rubidium, barium and thallium and potassium were used. A method for producing a second harmonic generation element, comprising forming an optical waveguide by performing ion exchange with the mixed molten salt of.